Any port that is neither a root port nor a designated port becomes a blocked port. Blocked ports do not forward user traffic. They only listen for BPDUs. This is how STP breaks loops. Imagine a triangle of three switches. Each switch has a root port pointing toward the root. Two segments have designated ports. The third link has one end designated and one end blocked. That blocked port prevents the loop. If the active path fails, the blocked port can transition to forwarding.
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$ curl repovive.com/roadmaps/network-design/spanning-tree-and-link-aggregation/blocked-ports
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